PROJECT SUMMARY/ABSTRACT Multicomponent behavioral and biobehavioral interventions, such as interventions aimed at preventing and treating substance abuse, HIV, and nicotine dependence, play a central role in public health. To date development and evaluation of interventions has relied primarily on the classical treatment package approach, in which a set of intervention components is identified a priori, assembled into a treatment package, and evaluated for effectiveness in a randomized controlled trial (RCT). Although the RCT is an excellent way to compare an intervention as a package to a suitable control or comparison group, use of the RCT alone does not enable addressing research questions that are critical for improving intervention effectiveness, efficiency, economy, and scalability. Recently an innovative alternative has emerged, called the multiphase optimization strategy (MOST). MOST is a broad methodological framework inspired by ideas that are fundamental and well-established in fields such as industrial engineering. MOST includes evaluation of an intervention in an RCT, but it adds an additional prior phase of research aimed at optimization of the intervention. To date, at least 30 projects using MOST have been funded by 10 NIH ICs, 6 of which participate in this RFA. There is considerable demand for training among both new and established intervention scientists who wish to develop intellectual grounding and a skill set in MOST. The purpose of the proposed work is to build investigator capacity in intervention optimization nationwide by developing a wide-reaching and sustainable training effort focused on MOST. Specific Aim 1 is to develop an online short course that provides a comprehensive introduction to MOST via videos and supporting materials, and make it freely available. Specific Aim 2 is to provide opportunities for intervention scientists who have completed the short course to gain additional conceptual and practical expertise needed to obtain NIH funding and apply MOST successfully in their research. These opportunities will be (a) three 2-day in-person trainings at various locations around the US, consisting of small group exercises, open discussion, a self-guided data analysis practicum, and individual consultation, and (b) follow-up individual consultation sessions. Specific Aim 3 is to foster a self-sustaining effort to build investigator capacity in MOST by providing (a) opportunities for 6 apprentice instructors to participate in the 2-day in-person trainings, (b) curriculum materials for a graduate course, and (c) individual consultation on course development. Specific Aim 4 is to develop a series of videos with supporting materials covering advanced, specialized, and emerging topics in intervention optimization and make them freely available online. The proposed work will increase the number of scientists proficient in and funded for optimization of behavioral and biobehavioral interventions. The work of these scientists will, in turn, produce more effective, efficient, economical, and scalable interventions, and thereby improve the nation?s health in substance abuse, HIV, cancer, and countless other important areas of public health.